Bushings of the foregoing character have, as those skilled in the art will appreciate, been well known and in extensive use for a number of decades. Indeed, dating back to at least the 1950s, conventional prior art walking beam bushings have been in widespread use employing inner and outer coaxial steel sleeves and an elastomeric rubber interface. Similarly, torque arm bushings have been employed during essentially the same period of time utilizing an outer cylindrical steel sleeve, an inner solid pin, and an elastomeric rubber interface. In all of such conventional heavy duty truck bushings, the elastomeric interface is substantially pre-loaded--that is, the elastomeric rubber interface is formed within the outer steel sleeve and then an oversized inner steel bushing member is inserted using presses capable of exerting several tons of pressure to form a finished bushing in which the elastomeric rubber material is heavily pre-loaded and is urged into intimate face-to-face contact with both the inner and outer steel bushing members; and, wherein the elastomeric rubber material fills the multiplicity of depressions and imperfections in the surfaces of the inner and outer steel bushing members. Moreover, in some extremely heavy duty applications, the suspension manufacturer will employ bushings of the foregoing character wherein the inner and outer surfaces of respective ones of the outer and inner steel bushing members are coated with a chemical or adhesive bonding agent to promote adhesion of the elastomeric rubber material to the steel.
Heavy duty truck suspension bushings of the foregoing character are still the bushings of choice by most OEM truck suspension manufacturers today; and, they even are employed as replacement bushings in a significant portion of the bushing aftermarket. Unfortunately, however, such conventional prior art bushings have an extremely limited life, resulting in limited lifetime warrantees and the necessity to rebush the vehicle's suspensions frequently. Indeed, it is common to have to replace such conventional prior art bushings annually, if not more often.
Bushings of the foregoing character have also been used in automotive suspensions for passenger cars, light weight and intermediate weight trucks, recreational vehicles, off-road vehicles, and the like; but, the bushings have continued to exhibit the same limited lifetime characteristics resulting in the need to frequently rebush. This is, of course, particularly true when the vehicle is extensively used over rough terrain and/or with heavy loads.
As a consequence, in the mid to late 1970s and continuing into the 1980s, a number of companies involved in the automotive bushing aftermarket have introduced urethane bushing interfaces which can be employed with the original inner and outer metal bushing members in lieu of elastomeric rubber material. In such aftermarket usage, the elastomeric rubber interface is removed from the metal bushing parts, the metal bushing parts are typically cleaned, and the preformed urethane bushing inserts are then pressed into place between the inner and outer bushing members. In some instances, the urethane, rather than being preformed into discrete elastomeric bushing inserts, is cast in situ between the inner and outer bushing members. Such urethane aftermarket bushings have, in recent years, begun to capture a significant portion of the automotive aftermarket; but, they still have not found widespread acceptance by OEM's who continue to employ bushings with elastomeric rubber interfaces despite the fact that the life of bushings employing urethane interfaces is significantly extended.
In 1989 U.S. Pat. No. 4,840,395 issued to George Sturmon and such patent has been assigned to Atro Engineered Systems, Inc. of Cuba, Mo. (hereinafter the "Sturmon '395 patent"). The Sturmon '395 patent purports to disclose bushings suitable for use with heavy duty truck suspensions and employing: i) a cylindrical outer metal sleeve; ii) an inner bushing member coaxial with the outer sleeve and defining an annular space therebetween; and iii), a urethane interface cast in situ into the annular space between the inner and outer bushing members--i.e., the urethane interface is not pre-loaded. The Sturmon '395 patent further discloses use of a chemical or adhesive bonding agent which is applied to the inner surface of the outer cylindrical sleeve so that, when assembled and cured, the urethane interface is chemically or adhesively bonded to the outer cylindrical sleeve while it is fully rotatable with respect to the inner sleeve.
Dyco Industries, Inc., the assignee of the present invention, has, in the past, made both heavy duty truck suspension bushings and automotive bushings of the foregoing character which were either: i) unbonded except for the frictional bond resulting from the substantial pre-load applied to the urethane and wherein end caps were employed to fix the components axially; ii) where urethane was adhesively bonded to the outer sleeve; or iii), where no adhesive bonding agent was employed but, rather, the urethane was fixed to the outer sleeve by the significant pre-load applied to the urethane as a result of forcing a 0.080" oversized inner bushing member into the outer sleeve/urethane subassembly under several tons of pressure. The latter two types of Dyco bushings have employed a ball and socket arrangement wherein a spherical socket is formed in the inner surface of the urethane and a complemental spherical ball, slightly oversized with respect to the socket, is formed on the inner bushing member. In all of the foregoing Dyco bushings, the significant pre-load resulted from pressing an oversized inner bushing member into the urethane/sleeve combination under several tons of pressure, served to mechanically fix the urethane interface with respect to the outer bushing sleeve, while the ball and socket connection has served to axially fix the urethane interface with respect to the inner bushing member while permitting freedom of relative rotation therebetween.
Bushings of the type disclosed in the Sturmon '395 patent, as well as those heretofore manufactured and offered for sale by Dyco, have represented an improvement over conventional prior art bushings principally in terms of improved life expectancy attributable to the use of a urethane interface in lieu of the more conventional rubber interface; and, this has enabled the bushing manufacturers to offer extended warrantees of up to three years. However, while such bushings have made significant advances in the aftermarket, OEMs have continued to resist use of urethane bushings in original equipment. It is believed that such resistance to the use of urethane bushing interfaces by OEMs is, at least in part, attributable to experience demonstrating that the adhesive bond between the urethane and outer sleeve does fail, particularly under heavy usage. Somewhat improved results have been achieved where the urethane interface is significantly pre-loaded; and, this has been true irrespective of whether a chemical or adhesive bond has been employed between the urethane interface and the outer cylindrical sleeve.
Another serious problem with conventional bushings of the foregoing character resides in their weight. For example, a typical walking beam center bushing employing inner and outer cylindrical steel sleeves and an elastomeric interface will commonly weigh on the order of 23 pounds, although those skilled in the art will appreciate that such bushings can weigh considerably more. Thus, for certain military applications, such bushings have weighed up to 240 pounds. Similarly, a conventional walking beam end bushing will typically weigh 7 pounds, while a conventional torque arm bushing will commonly weigh on the order of about 4 pounds. Considering typical heavy duty truck suspensions, such suspensions will normally employ two (2) walking beams requiring two (2) center bushings (total bushing weight is approximately 46 pounds), four (4) end bushings (total weight is approximately 28 pounds), and anywhere from four (4) to ten (10) torque arm bushings (total weight ranging from about 16 pounds to about 40 pounds). Thus, the total weight of simply the bushings in a heavy duty truck suspension will be, on average, somewhere between 90 pounds and 114 pounds. And, of course, it is known that every pound required in the vehicle suspension system results in one less pound in the vehicle's load carrying capacity. Since it follows that for every pound that can be eliminated from the truck suspension, the load can be increased by a pound for every trip, it is extremely important to the OEMs to find ways that the weight of the truck's suspension can be reduced, thereby increasing load carrying capacity. Yet, notwithstanding the foregoing, for a half century or more, there has been no significant weight reduction in truck suspension bushings.